The present invention relates to the devices used to anchor structural cables used in construction work. It applies in particular to stays, pre-stressing cables and suspension cables of suspension bridges.
The stays are cables generally designed to transmit tensile loads between two points of a structure to which they are anchored. They are therefore in theory straight, if external effects which tend to curve their path are neglected.
The catenary effect due to the self-weight of the stay, the effect of the wind (external transverse pressure), the slight rotational movements of the building elements supporting the stay anchors, the effects of variations in temperature are factors which lead to angular deflections at the ends of the stays, i.e. where they emerge from the anchor points.
In other cables, significant deflections as they emerge from the anchor point are also possible because of the line they are forced to follow or because of transverse action to which they are subjected.
The construction of anchor points is generally such that only tensile loading is reacted satisfactorily. Local bending moments brought about by the abovementioned angular deflections that may be applied to the anchor point are filtered by means of a continuous or insulated guide at the anchor point exit and located a suitable distance away to ensure that they are sufficiently effective.
The principle of anchoring is based on the individual wedging of each of the tendons of which the cable is made. This entails a certain transverse spacing of the tendons at the anchor block so as to have enough space to fit the individual wedging means which are generally jaws with frustoconical wedges.
In the case of stays, a deflector brings the tendons together into a compact arrangement a certain distance away from the anchor point so as to minimize the overall cross section of the stay in the running part. In general, the guide which filters out the bending moments lies at the deflector which collects the tendons together into a compact formation (see, e.g., EP-A-0 323 285). The relatively long distance between the guide and the anchor block (typically more than one meter) is needed to limit excessive angular deflections of each tendon which would carry the risk of damaging it and would result in additional bending moments at the anchor block. In addition, taking up bending moments too close to the anchor point would leave significant transverse loadings at the anchor block.
GB-A-2 157 339 discloses a stay anchoring device wherein a deflector is mounted in two parts in a tube secured to the anchor block. The part furthest from the anchor block prevents contact between the external strands and the tube, while the part closest to the anchor block prevents the strands from rubbing together when cyclic loadings are applied to the stay. The bending moments, to which the document pays no particular attention, are essentially reacted at the part of the deflector furthest from the anchor block.
In other arrangements, the stay downstream of the anchor block passes through an orifice which widens toward the running part, and which allows the whole of the stay an angular deflection by reacting the bending moments along the length of the zone over which the stay bears against the orifice (see, e.g., GB-A-2 097 835).
An object of the present invention is to propose an anchoring system which limits the bending stresses of the cable to permissible value as soon as the cable leaves the anchor point. Another object is possibly to make it possible to dispense with an additional external device for reacting the bending moments that are due to the variations in the path of the cable.
The invention thus proposes a device for anchoring a structural cable, comprising an anchor block having orifices therethrough, each accommodating a tendon of the cable and a means of immobilizing said tendon, a bearing piece for the anchor block, and means of guiding the tendons between the anchor block and a running part of the cable, wherein the guide means are connected to the bearing piece and comprise an individual guide passage for each tendon of the cable. Each guide passage widens toward the running part of the cable so as to allow angular deflection of the tendon accommodated in said passage. The guide passages have, in the direction of the anchor block, a transverse layout aligned with that of the orifices in the anchor block.
The overall design of the anchor point is greatly simplified by associating the guide means directly with the anchoring device. The tendons of the cable are individually guided, which means that the inertia of the flexing element is significantly lower than the overall inertia of the cable. This results in effective filtering of the bending moments at the anchor block, even if the distance between the anchor block and the guide means is relatively short. Individual guidance of the tendons avoids the cumulative effect of the transverse loads of the layers of tendons on one another.
Advantageously, each guide passage widens toward the running part of the cable with a radius of curvature that is substantially constant in a plane passing through the axis of said passage.
In a preferred arrangement of the device, the guide means comprise at least one guide member housed in a tube connected to the bearing part, through which the tendon-guiding passages are formed.
The guide member may lie just behind the anchor block, or be spaced a certain distance away from the anchor block. In the latter case, it is possible to make provision for the tendons of the cable to be strands individually protected in the running part, the individual protection of each tendon being interrupted in a chamber lying between the guide member and the anchor block, with sealing means placed between said chamber and the guide member so as to form a sealed separation between the chamber and the running part of the cable, and to contain a filling and protective product injected into the chamber. The device possibly comprises a second guide member lying between the anchor block and the sealing means.
The guide member may be made of a rigid or deformable material. In the latter case, it is advantageous to leave a clearance, in the direction of the running part of the cable, between the circumference of the guide member and the tube in which it is housed, so as to allow the collection of tendons of the cable an angular deflection by deformation of the material of the guide member. The shape of this clearance is optimized so as to provide uniform curvature. When the guide member has a cylindrical periphery, the clearance may result from a widening of the inner face of the tube toward the running part of the cable, with a radius of curvature that is substantially constant in a plane passing through the axis of the tube. When the tube has a cylindrical inner face, the clearance may result from a narrowing of the periphery of the guide member toward the running part of the cable, with a radius of curvature that is substantially constant in a plane passing through the axis of the tube. Another possibility is that the clearance results partly from a narrowing of the periphery of the guide member toward the running part of the cable and partly from a widening of the inner face of the tube toward the running part of the cable.
Advantageously, the deformable guide member has a viscosity, so as to damp the cable when the latter oscillates. This viscosity may be intrinsic to the deformable material of the member and/or may result from a viscous substance contained in cavities formed in this member.
The deformable guide member may comprise, between the guide passages, inserts of an inertia that decreases toward the running part of the cable, which makes it possible to control the curvature experienced by the cable through the member. As an alternative, the tube in which the deformable guide member is housed may have an inertia that decreases toward the running part of the cable.